In the folding paper box machine art, sheet feeders are well known, and it has heretofore been proposed to use feed control apparatus, also known as "bumper" type feeds, in the feeding zone for assuring that only the lowermost blank in a stack of identical flat box blanks will be advanced through the gateway of the magazine and along the horizontal paper line of the machine.
One type of box blank feed is disclosed in U.S. Pat. No. 4,200,033 to Raymond A. Labombarde of Apr. 29, 1980 which has a mechanically actuated timed bumper type magazine feed which feeds flat box blanks onto an untimed conveyor in a flap feeding zone. This bumper feed has elongated lifter bars mounted in the space between feed belts which are raised and lowered to control the feed of each successive lowermost blank out of the gateway at the bottom of the stack.
In another type of bumper feed disclosed in U.S. Pat. No. 3,612,512 to Lang of Oct. 12, 1971, the lifting member is between the carrier belts and differs from applicant's structure because it is in pivotal cooperation with the blanks.
This structure presents a problem for high speed feeding in that as the lifter is lifted upwardly, it may cause friction between the gate and the lift plates acting on the trailing end of the carton being fed, and cause some hesitation or marking on the carton, and therefore, the dwell period of the lifting has to be accurately timed by means of a cam to suit each length of carton being fed.
This feed is not capable of running paper or thin material, as compared to regular boxboard, nor is it suitable for cartons or paper that has a high quotient of friction, as the friction between the sheets tends to pull the second sheet into the nip.
Furthermore, this feed is therefore sensitive to the pile height in the feed hopper, whereas applicant's new feed overcomes this difficulty.
Another type of bumper feed is disclosed in U.S. Pat. No. 3,406,963 to Goss of Oct. 22, 1968, wherein the upper reaches of a plurality of endless carrier belts advance under the lower surface of the lowermost blank in a stack and are liftable into contact with that surface to advance the lowermost blank by means of cams.
The applicant's invention has many advantages over the prior art "bumper" type feed apparatus including the following advantages.
Because the feed nip between the feed belt and gate is completely isolated from the stack of sheets during the nonfeeding portions of the feed cycle, the next blank is prevented from creeping into the feed out of time.
Some sheets are difficult to feed because of the high coefficient of friction between the sheets and, therefore it is advantageous to have the feed stack at a high angle to allow the traveling portion of the sheet being fed to fall free from the rest of the stack to reduce frictional drag. This is the only "bump" feed that can be used in this manner since a high angle on all other bump feeds will cause false synchronization feeding, due to the proximity of the leading edge of the lowermost sheet to the "nip point".
When attempting to feed thin sheets such as paper (such as 0.003 inch thick) on a standard bottom feed it is particularly difficult and often impossible to prevent the second sheet from being urged forwardly as soon as any gripper surface touches the sheets prior to the first sheet leaving the feed gate.
When this happens some sheets tend to wrinkle thereby causing a jam. With this new feed, however, forwardly urging or jamming of the next sheet being pre-fed by the gripper face is eliminated since the stack lifter means lifts the stack away from the gripper surface sufficiently in advance, to eliminate possible contact of the second sheet with the gripper surface, while the first sheet is being fed.
The lifting of all or at least the front end of the stack during the "no feed" cycle and allowing the stack to engage the gripper surface at the maximum descending speed of the eccentric increases the effectiveness of the "gripper surface" to advance the blank at precisely the correct moment. This "slamming down" of the stack onto the feed belt at the feed nip position is only possible using this invention, and therefore, an effective contribution to steady, high speed productive feeding.